THK u\i.\ iNsj KI MKNTS of note that have been proposed to sup- 

 plement the transit, level and alidade for topographical surveys 

 are the photo-theodolite which, in expert hands, is more accurate 

 and more thorough than the plane table and the telemeter which 

 provides the most rapid known method of obtaining approximate 

 distances. 



For military work the telemeter is known as the Range Finder. 

 There are various types but generally the basic optical principles 

 are alike. The instrument contains an optical base-line with suit- 

 able prisms to project two images of the field upon a coincidence 

 plane in the focus of the eyepiece. The amount of deflection 

 required to perfect a continuous and symmetrical image of the 

 object sighted, depends upon its distance from the instrument. 



Naturally the longer the optical base the greater the possibilities 

 for accurate and consistent observations. Those built for the Navy 

 or Coast Defence and designed for permanent mounting attain the 

 length of 30 ft. and over and reduce errors to less than one percent. 

 Where portability is the first consideration, as for Infantry use, the 

 instrument is reduced in length to 80 cm and in weight to about 

 5 kg. In this type the effective range lies between 400 and 5000 

 yds. which is sufficient for the required pui 



Range Finders for the shoitei i!i --tunces encountered in topog- 

 raphy would have to be construcu-d in a still shorter base \\\\\\ 

 increasing probabilities of error that doubtless would exceed those 

 DOW attributable to stadi.i The larger models have self 



contained triple mirror adjusting apparatus, astigrnati/er for o! 

 ing luminous spots at night, sun light filters and variable magniri- 

 i The system of collimation must be very exact. It has 

 been found advantageous to mount the optical system, with objective 

 penta prisms, on wires under high tension; not only to preserve 

 perfect alignment but to overcome flexure and add stiffness to the 

 entire structure. 



